Apparatus for making plastic film

ABSTRACT

Plastic film is made using a heat transfer liquid circulated through the film extrusion die to control (by either heating or cooling, as required) the temperature of the plastic melt flowing through such die. A plastic bubble emerging at the outlet of the die is inflated and ventilated by continuously circulated gas which is controlled in temperature as it enters and leaves the bubble so that it does not adversely affect the temperature of the melt in the die.

United States Patent Hinrichs 1 1 Aug. 14, 1973 [54] APPARATUS FORMAKING PLASTIC FILM 3,266,092 8/1966 Corbett 425/462 1 1 Inventor:Donald Hinflche Richmond, 313331333 1313?? i;12E..f.f.:.::: "63111252141? 0 Va. 3,525,125 8/1970 Berger et a1. 425/457 [73] Assignee: ReynoldsMetals Company, v

4 Richmond v Primary Examiner-J. Spencer Overholser AssistantExaminer-Ben D. Tobor [22] filed: 1971 Attorney-Glcnn, Palmer, Lyne,Gibbs & Thompson [21] Appl. No.: 126,831

I a ,2 1 [57] ABSTRACT 1 US. 25/ 6 425/1 1 /326, Plastic film is madeusing a heat transfer liquid circu- 25/379 lated through the filmextrusion die to control (by ei- [51] Int. Cl. B29c 1/00 ther heating orcooling, as required) the temperature of [58] Field of Search 425/197,461, 467, the plastic melt flowing through such die. A plastic 425/326,379 bubble emerging at the outlet of the die is inflated and ventilatedby continuously circulated gas which is con- [56.] 1 References Citedtrolled intemperature as it enters and leaves the bubble UNITED ST 1155PATENTS so that it does not adversely affect the temperature of2,937,402 5/1960 Pierce 425/2015 the melt the 3,051,989 9/1962 Mercer425/465 17 Claims, 9 Drawing Figures 3,114,169 12/1963 Palmer et a1.425/198 X 3,281,897 11/1966 Mercer 425/380 Patented Aug. 14, 1973APPARATUS FOR MAKING PLASTIC FILM BACKGROUND OF THE INVENTION Plasticfilm has long been made by inflating a tubular section being extrudedfrom an annular orifice in a die to define a tubular film bubble andthen collapsing the tube between a pair of cooperating rollers to definea double thickness film which is rolled on a take-up roll. Control ofthe temperature of a plastic melt flowing through an extrusion die hasbeen a problem. Degradation of the melt within the die or buildup ofsolidified melt along the inner lip of the die outlet can have adverseeffectson film quality and shorten the operating cycle of the die beforethe die must be shut down for recleaning of the die passages.Temperature control is important for achieving uniform gauge across thewidth of the film strip after slitting of the extruded tube. Any lack ofuniform gauge is magnified when the film is subsequently stretched, asit frequently is for purposes of making oriented heat-shrinkable film.

. SUMMARY This invention provides an improved apparatus and method forextruding plastic film in a more economical manner, due to longeroperating cycles between shutdowns, and with improved uniformity ofgauge transversely of the direction of extrusion. This apparatus andmethod employs a temperature and flow controlled heat transfer liquidwhich is circulated through the extrusion die to control (by eitherheating or cooling, as required) the temperature of the plastic meltflowing through the die. Air or other gas used to inflate and ventilatethe plastic bubble emerging from the die outlet is also controlled intemperature when it passes through the die so as to not adversely affectthe temperature of the melt in the die. e

Other details, uses, and advantages of this invention will becomeapparent as the following description of the embodiment thereofpresented in the accompanying drawings proceeds.

BRIEF DESCRIPTION OF THE DRAWINGS The accompanying drawings show apresent preferred embodiment of this invention, in which FIG. 1 is aschematic side elevation, partly sectioned and partly broken away, of afilm extrusion die and associated temperature control apparatus,including the emerging tubular film bubble and pinch rolls;

FIG. 2 is a'perspective view schematically illustrating the overall flowof fluids through the extrusion die of FIG. 1;

FIG. 3 is a schematic and partially broken away fragmentarycross-section view taken on the axis of the film extrusion die shown inFIG. 1;

FIG. 4 is a cross-sectional view taken essentially on the line 4-4 ofFIG. 1;

FIG. 5 is a schematic and partially broken away fragmentarycross-section view taken approximately on the line 5-5 of FIG. 4 to showthe flow passages of the liquid and gas between the channels carryingthe plastic melt;

FIG. 6 is an enlarged fragmentary perspective view particularlyillustrating the detailed construction of a stranding chamber and acombining chamber comprising the exemplary extrusion die of thisinvention;

FIG. 7 is a fragmentary cross-sectional view taken essentially on theline 7-7 of FIG. 4;

2 FIG. 8 is a greatly enlarged fragmentary view particularlyillustrating the cross-sectional configuration of a part of thestranding chamber; and

FIG. 9 is a view similar to FIG. 8 of the combining chamber.

DESCRIPTION OF ILLUSTRATED EMBODIMENT The apparatus shown in FIG. I ofthe drawings is designated generally by the reference numeral 10 and isadapted for extruding polyvinyl chloride, nylon, or other polymeric orlike plastic material to produce a thin plastic film.

The apparatus 10 comprises an extrusion die I] having an inlet 12 and anannular outlet 13 and an extruder 14 supplies a hot plastic melt 15under pressure to the inlet 12. A supply bin 16 feeds the extruder 14with a suitable plastic material 17 in a flowable particulate form; and,the extruder 14 has heating means for melting the plastic material 17and a helical screw 20 for forcing the molten material or melt 15through an outlet 21 and through a supply conduit 22 to the die inlet21. The plastic melt 15 is supplied to the inlet 12 at a controlled andsubstantially constant temperature suitable for the physical andchemical properties of the plastic material 17, i p

The apparatus 10 has means indicated generally at 23 for supplying airor other suitable gas (designated by dotted arrows 24) through the die11 to inflate a tube produced by extrusion of the plastic melt 15through the annular die outlet 13, and such tube is pinched by rollers26 to trap a moving film bubble 25 between the die outlet and therollers. The double thickness of film passing from the rollers 26 may bewound on a supply roll or further processed in any suitable manner knownin the art. The tube is slit and the film is stretched to increase itsarea and then chilled to retain it in stretched condition. It is then incondition for subsequent heat shrinking for packaging purposes.

The air 24 is suitably supplied to the die 11 and is conveyed axiallytherethrough using circumferentially spaced passages 27, see FIG. 3. Theair is then conveyed by radially inwardly directed passages to a centralcylindrical passage 30 which extends substantially axially through theupper end portion of the die 11 into the film bubble 25. The air 24 issupplied at a controlled temperature and pressure to cause expansion ofthe film bubble 25 in a precisely controlled manner.

The air 24 flows into the center of bubble 25 through passage 30 andinitially flows upwardly into such bubble as indicated by the arrows at31. It then flows downwardly along the sides of the bubble (as indicatedby the arrows 32) into a plurality of passages 33 through the die whichextend in spaced parallel relation to each other and are arrangedconcentrically around the central passage 30. The passages 33communicate with an annular chamber 34 and one or more passages 36 areprovided in die 11 in flow communication with chamber 34 for conveyingthe air in such chamber out of the die.

Volatile substances from the plastic melt pass into the air in thebubble, and tend to build up in concentration as more film passes aroundthe bubble. Moreover, cold air is blown (by conventional means, notshown) against the outside of the emerging extruded film to chill andsolidify it. These effects canlead to condensation of the volatilesubstances and consequent damage to the film upon being contacted by thecondensate.

Continuous withdrawal of air in the bubble 25 prevents such build up ofconcentration of volatiles and, thus, protects the film from thedamaging effects of excessive concentration of volatiles.

The apparatus and method of this invention uses a 5 parts of the die. Inaddition, the liquid 35 is also used (as will be describedsubsequently), to control the temperature of the air 24 entering andleaving the bubble 25 to further assure precise control of thetemperature and, hence, even flow of the melt through the die 11.

The liquid 35 is supplied to the die from a manifold 123 which encirclesthe die.

While the liquid 35 might for heating purposes be replaced by fluids inother phases, such as gases or gaseous vapors (e.g., steam), a liquid isfar superior because it can function efficiently both for heating andcooling, which is part of the intended operation of the apparatus andmethod of the invention. Steam and like vapors may be suitable forheating but are unsuitable for cooling in dies of the kind in question.Gases like air are inefficient for both heating and cooling in dies ofthe kind in question, because of poor heat absorption and heat transfercapacity.

During operation of the apparatus 10, the melt 15 enters inlet 12 andtravels vertically upwardly through a cylindrical passage 44 in a lowerportion 11A of die 11.

The melt I5 flares outwardly from the top of passage 44 into an annularconically extending passage 45 in the die lower portion 11A. The die IIhas a stranding chamber 46 in flow communication with the peripheraledge of the passage 45; and a plurality of spaced parallel channels 47extend through a central portion 11C of die 1 l and at their lower endscommunicate with the peripheral edge of the stranding chamber 46 andconvey the melt therefrom in a plurality of separated plastic strands 48through the central die portion 11C. The

strands 48 are recombined in a combining chamber 49 in the upper portion113 of the die where the upper ends of channels 47 communicate withchamber 49.

an annular cylindrical passage 50 through the upper die portion 118. Theupper end of passage 50 terminates in the annular die outlet orifice 13.

Die 11 has integral heat transfer means comprising a plurality ofcooperating passages in the die body. The lower portion IIA of the die11 has passages 51 to provide circulation of liquid 35 closely adajcentthe melt passing into and through conical passage 45. A plurality ofinterconnected passages 52 provide flow of liquid 35 through parts ofthe central die portion 1 1C and circumferentially around portion Cwhere it extends around the outside of the strand channels 47. Otherinterconnected passages 53 provide circulation of liquid 35 through theupper die portion 118 where it extends around the outisde of the annularpassage 50. In addition, interconnected passages 54 provide flow ofliquid 35 through the central and upper die portions 11C and 11B andhave portions which pass closely adjacent to the strand channels 47 andannular passage 50. The passages 54 also extend closely adjacent toreturn airconveying passages 33. The interconnected passages 54 of thisexample are placed in flow communication with the passages 52 throughthe use of radial passages ....--Theohamber 49 communicates withthedewec end of.

55. The passages 55 extend radially through the central die portion 11Cand provide flow of liquid 35 between pairs of strand channels 47 andboth to and from passages 54. The liquid 35 is collected from the die inmanifold 124 which encircles the die.

The liquid-conveying passages 51-55 have been shown schematically inFIG. 2; and, it will be appreciated that this has been done forsimplicity of presentation. Further, in some applications it may bedesired to provide liquid to the passages 54 independently of thepassages 52.

FIG. 5 shows a schematic arrangement of air and liquid passages in thedie as air and liquid pass continuously between the strand channels 47.In the present preferred embodiment of the invention, a series of strandchannels designated 47A, 47B, 47C and 47A have different flow passagesfor air and liquid between them, and the flow passage pattern isrepeated for each succeeding set of four channels (channel 47A being thefirst channel of the next series of four) and so on around the die.Between channels 47A and 478 there is a radially extending air outflowpassage (indicated by flow arrows designated 36A) connecting passages 33and 36. About midway between channels 478 and 47C there are radiallyextending liquid outflow and inflow passages (indicated by flow arrows55). The liquid outflow and inflow passages are displaced vertically toavoid interference with each other. Incoming preheated air 27A is fedbetween channels 478 and 47C (shown close to channel 47C for conveniencein the figure, but actually midway between channels 478 and 47C anddisplaced vertically to avoid interference with the liquid passages 55).The arrangement of air and liquid passages between channels 478 and 47Cis repeated between channels 47C and 47A. The pattern is repeated forthe next series of four channels as explained above, wherein 47A is thefirst channel of thenext series. Y

The liquid 35 thus passing back and forth between strand channels 47controls the temperature of the metal between strand channels 47 tothereby control the temperature and flow of the melt 15. The liquid 35also flowing through those portions of passages 54 arranged adjacent airpassages 33. supplementsthere;

heating of cooled returning bubble air in passages 33 for purposes ofsuppressing undue local cooling effect on the melt of the returning airas it passes between strand channels 47.

The incoming air 24 provided to the die 11 is preheated to a temperatureclose to the temperature of the melt and the liquid 35 flowing throughthe various in terconnected liquid-conveying passages 51-55 assures thatthe preheated air temperature is equalized with that of the melt as theyboth flow through die I 1. In addition, these liquid-conveying passagesare supplied with liquid 35 at the same predetennined temperature,preferably by being connected to a common preheated reservoir and thistends to equalize temperatures throughout the die.

In extruding melt through an associated die the temperature of the meltadjacent the die outlet is greater than the temperature of such melt atthe die inlet. To prevent the melt outlet temperature from becomingexcessive, which would cause rapid degradation and pos sible charring ofthe melt resulting in blockage of the melt passages, the temperature ofthe liquid 35 flowing through passages 53 and 54 in the upper dieportion 11B, for example, is relatively cooler than the temperature ofthe melt as it flows through die portion 118 whereby such liquid servesto locally cool the melt.

The exemplary apparatus has its supply of heat transfer liquid 35contained in a reservoir 56, see FIG. 1, and a system is provided forsupplying the liquid 35 to the die 11 and returning such liquid to thereservoir 56 and such system comprises a supply conduit system 57, areturn conduit system 58, and suitable pressure means in the form of apump 60 driven by a motor 61 for flowing or circulating the liquid 35through the conduit systems 57 and 58, and, hence, die 1 1. Theapparatus 10 also has a heat transfer assembly 62 which is provided withheat exchangers and suitable controls for controlling the temperatureand flow of liquid 35.

The air 24 supplied to the die 11 for inflating the bubble 25 ispreheated in a heat exchanger 64 which is supplied with liquid 35 at acontrolled temperature from the assembly 62. The heat exchanger 64 is inthe form of an oil-to-air heat exchanger and is supplied with air from acompressor 65. However, the air 24 may be preheated using any suitabletechnique or may be partially heated prior to being introduced into theheat exchanger 64. An adjustable air pressure regulator 66 is alsoprovided in a supply conduit 67 which provides the air 24 to the inletpassages 27 of the die and the regulator 66 may be adjusted to assureproper inflation of the bubble 25.

In this example, liquid 35 at a predetermined temperature and fromreservoir 56 is supplied to all parts of the die 11. The die 11 may besupplied with suitable external conduits so that the flow of liquidthrough its portions 11A, 11B, and 11C may be in series, parallel, or inseries through certain die portions and in parallel through the otherportion and vice versa. In addition, the use of parallel flow pathsthrough the die makes it possible to supply liquid at differenttemperatures and, if desired, from separate reservoirs through each flowpath whereby in one portion of the die the liquid 35 provides heatingwhile in another portion of the die such liquid provides cooling.However, regardless of whether a liquid 35 is supplied to each dieportion from a single reservoir or a plurality of reservoirs, by havingparallel flow paths through the die the rate of liquid flow through eachparallel path may be controlled by the configurations and sizes of theflow passages and the use of suitable valves and restrictors whereby thedesired amount of heating or cooling provided by the liquid 35 in theassociated die portion may be precisely controlled.

To enable easy comprehension of the overall operations of the die 11 theabove description has been made referring to only FIGS. 1 and 3 of thedrawings. The presentation will now proceed with a more detaileddescription of the construction and operation of the component partscomprising the exemplary die 11.

3 As best seen in FIG. 4, the die 11 has a fixed lower or bottom portioncomprised of a die stator 70 and a slip ring assembly 73, to besubsequently described in more detail, which allow rotation of theentire remaining upper portion of die 11 relative to the nonrotatingfilm bubble 25 and this compensates for any irregularities or defectswhich might be present in the annular orifice 13, for example, therebyassuring that rolls wound from the film will have uniform thicknessthroughout. Uneven flow of the plastic melt through the die 11 issubstantially eliminated due to the unique design of its melt flowpassages and the precise control of the temperature of the flowing melt.

The die stator has a catch basin 71 suitably fixed thereto by bolts 72.The slip ring assembly 73 is comprised of a central slip ring 74, drivebars 75, and an upper segmented ring 76, with the members 75 and 76being fixed to the central ring 74 by bolts 77 and 78 respectively.

The stator 70 and assembly 73 are restrained from rotating by a suitableframe or supporting structure and the heat transfer liquid 35 for thedie 11 is provided through an inlet 79 in the stator 70 whichcommunicates with an annular chamber 79A. The air 24 used to expand thebubble 25 is provided to the die 11 through an inlet connection 27A inthe assembly 73 which supplies air to an annular chamber 278 and thepreviously described passages 27 are in :flow communication with chamber273. With this arrangement it is possible to provide continuous rotationof the upper part of the die 11 without concern for the air and heattransfer liquid connections.

The die 11 has an insert and hub portion designated generally by thereference numeral 80 which is comprised of a central member 81 which haspassage 44 extending axially therethrough in the form of a rightcircular cylindrical opening. A member 82 is fastened concentricallyaround member 81 to define an annular cylindrical passage 83 used toconvey liquid 35 closely adjacent the column of melt 15 'in passage 44.The lower portion of the member 82 is suitably threadedly fastened to aring 84.

A die rotor 85 is threadedly fastened to the member 82 as indicated at86 and the'die rotor 85 with the insert and hub assembly 80 fixedthereto is provided with a rotary bearing assembly 87. The upper part ofdie 11 may be easily rotated (while keeping the die stator 70, catchbasin 71, and assembly 73 stationary, as previously mentioned) merely byrotating member 82 through a threaded connection 90 provided in thelower portion of the die 11. Although means have not been shown forrotating the upper portion of the die 11, it will be appreciated thatany suitable means known in the art may be used for this purpose.

Suitable fluid seals are also provided, as shown in FIG. 4, to assurethat neither air 24 nor heat transfer liquid 35 leaks out of the die 11between the die stator 70, assembly 73, and adjacent rotatingcomponents.

The die ll has what will be referred to as a die base 91 suitably fixedto the die rotor 85 and, hence, assembly 80 by a plurality of bolts 92and roughly frustoconical female surface 93 defines the top surface ofthe die base 91. The surface 93 cooperates with a substantiallyfrustoconical male surface 94 comprising a member which will be referredto as a mandrel shell 95 to define the previously mentioned passage 45.The mandrel shell 95 comprises a part of both the upper die portion 11Band the central die portion 111C.

The die 11 has a member 96 supported in the central portion of themandrel shell 95 and member 96 comprises the upper portion 118 of suchdie and has the passages or bores 33 defined therein. The member 96 alsohas the right circular cylindrical air passage 30 defined substantiallycentrally therethrough and the passages 33 are arranged in spacedrelation concentrically around the central passage 30, see FIGS. 3-5.The member 96 is held in position by a mandrel cap 100 which is attachedto the mandrel shell 95 by a plurality of threaded bolts 101. Suitablefluid sealing rings are provided between the members 95, 96 and 100 toassure that the heat transfer liquid 35 and air 24 do not leak into eachothers passages.

The die 11 has a pad ring 102 supported concentrically around the lowerportion of the mandrel shell 95 so as to define the annular cylindricalvolume or passage 50 therebetween; and, the die 11 also has a cup ringassembly 104 comprised of a top cup ring 105A and a bottom cup ring 105Bfixed together by a plurality of bolts 106. The assembly 104 is fixed tothe die rotor 85 by a plurality of bolts 107 which have plain centralportions of extended lengths and threaded end portions. Each centralportion of each bolt extends through aligned bores 108, 109, and 110 inthe die rotor 85, die base 91, and mandrel shell 95 respectively whichhave larger diameters than the outside diameter of an associated bolt107 whereby an annular air discharge passage 36 of extended length isdefined between each bolt 107 and its associated aligned bores.

The die 11 has radially extending air discharge passages 36A in member95, which are shown by dotted lines in FIG. 4 and schematically in FIG.5, and such passages communicate with chamber 34 and the upper ends ofpassages 36. The lower ends of air passages 36 communicate with anannular chamber 363 through radially extending passages and an outletconnection 36C is provided for discharging the air 24 out of the die.

The threaded end portions of bolts 107 are threadedly fastened in thebottom cup ring 1058. Thus, it is seen that the top outer portion 11B ofthe die is held to the die rotor 85 by bolts 107 and unique airdischarge passages 36 are provided around the outside surfaces of suchbolts.

The top cup ring 105A has an inside cylindrical surface 112 whichcooperates with an outside cylindrical surface 113 of the mandrel shell95 to define the upper portion of annular passage 50 and such upperportion terminates in the annular die orifice 13.

A header ring 115 is provided at the top of the die 11 and is fixed inposition at the top of and concentrically around the shell 95 bywelding. The previously mentioned interconnected passages 53 areprovided in the assembly 104 comprising the upper die portion 113 andcirculate heat transfer liquid 35 adjacent the outside surface of themelt as it is extruded in tubular form through the passage 50.

The die 11 has a manifold assembly or manifold 120 comprised of a ring121 having a roughly T-shaped cross-sectional configuration and an outercylindrical ring 122 which is fixed to ring 121 in a fluid-tight manneras by welding, or the like. The manifold 120 is held in position betweenthe die base 91 and the cup ring assembly 104 and suitable fluid sealsare provided between the mandrel shell 95 and manifold 120 to preventescape of liquid 35 from the manifold 120.

The manifold 120 has a pair of substantially toroidal chambers 123 and124 provided therein which provide flow of heat transfer liquid 35circumferentially around the central die portion 11C. The chambers 123and 124 communicate with passages 123A and 124A respectively and thechambers 123-124 and passages 123A-124A comprise the previouslydescribed interconnected passages 52.

The previously mentioned radially extending liquid conveying passages 55communicate with passages 123A and 1248 and assure that liquid 35 isprovided adjacent each strand channel 47. In particular, reference ismade to FIG. 5 of the drawings where it is seen that a plurality of fourstrand channels 47 have been shown defining three zones X, Y, and Zbetween immediately adjacent pairs of channels. Heat transfer liquid 35flows both inwardly and outwardly between channels 47 defining zone Xwith air 24 being supplied from passage 27 through radial passages 27C,also see FIG. 4, and into passage 30. The flow of liquid 35 and air 24between the channels 47 defining zone Y is similar to the flow in zoneX. Only air flow is provided between the channels 47 defining zone Z andsuch air 24 is from the bubble 25 and has passed through passages 33 andchamber 34 whereupon it is conveyed by radial air passages 34A to airdischarge passages 36. Following zone Z, zones X, Y and Z are repeatedaround the entire circumference of the die.

As will be apparent particularly from FIGS. 2 and 4 of the drawings, thepassage 45 is provided with a plurality of radially inwardly directedpassages 125 adjacent thereto which are angularly spaced only a fewdegrees apart, essentially as shown in FIG. 2. The passages 125 aresupplied with liquid 35 from an annular chamber 126 which is in turnsupplied with liquid from inlet 79. The passages 125 communicate with achamber 127 which supplies liquid to passage 83. The passages 83 and 125together with chambers 126 and 127 comprise the plurality ofinterconnected passages 51 previously described in connection with H6. 3and such interconnected passages assure precise temperature control ofthe melt 15 in passages 44 and 45.

As previously mentioned, the die 11 has a stranding chamber 46 and acombining chamber 49 and such chambers are arranged at opposite ends ofthe strand channels 47, see FIGS. 6-9. The chamber 46 is defined bycooperating surfaces in the die base 91 and the mandrel shell 95. Thechamber 49 is defined by cooperating surfaces in the mandrel shell andthe lower inside portion of the pad ring 102. The chambers 46 and 49 arevery similar and fragmentary portions of these chambers are shown inFIGS. 8 and 9 respectively; therefore, similar component parts thereofwill be given identical reference numerals and subsequently describedonly in connection with the stranding chamber 46 with it beingunderstood that a similar description is fully applicable to thecombining chamber 49.

The chambers 46 and 49 help assure smooth flow of viscoelastic materialthrough the die at a substantially constant shear rate, even though themelt 15 is being stranded or separated into a plurality of spacedstrands in the chamber 46 and combined in a tubular form in chamber 49.The flow of melt 15 at a substantially constant shear rate is also madepossible through the use of the integral heat transfer passages'providedin the die 11 which assure that the melt flowingthrough such die is keptat a substantially constant temperature by the action of the liquid 35providing either heating or cooling, as required, and by heating the air24 which flows out of the bubble 25 to prevent cooling of the plasticmelt by such air as it exits the die 11.

Each chamber 46 and 49 has a pair of opposed walls or surfaces 133 and134 and those portions of walls 133-134 which are arranged nearer thecenter of the die 11 are arranged approximately horizontally and insubstantially parallel relation. The chamber 46 also has an undulatingperipheral surface which is designated generally by the referencenumeral 131 in FIG. 6 and extends approximately vertically between walls133 and 134. The surface 131 is defined by a plurality of spikes orsubstantially V-shaped surfaces 135 interconnected at their bases byarcuate surfaces 136 which may be substantially semicylindrical in someapplications.

The V-shaped surfaces 135 are provided with a fillet 137 extending in asmooth arc'between surface 133 and surface 134. A fillet 137 is alsoprovided between arcuate surface 136 and surface 133 and in this latterinstance, such fillet has a substantially elliptical cross sectionalconfiguration. The fillet 137 further assures smooth flow of meltthrough its associated chamber. As will be apparent from FIG. 6, each ofthe surfaces 136 is aligned with an associated roughly semicylindricalwall portion of a strand channel 47 whereby the projections 135, arcuatesurface 136, and fillet surfaces 137 provide smooth converging flow ofmelt from stranding chamber 46 into the strand channels 47. Similarsurfaces in the combining chamber 49 provide smooth diverging flow ofthe melt into the annular passage 50 which communicates with thedischarge orifice 13.

The spaced bores 47 preferably have circular crosssectionalconfigurations' throughout their lengths and, in this example, each ofthese bores has a tapered or frustoconical configuration. Thefrustoconical configuration may be precisely finished using taperedhelical reamers to provide a smooth surface which assures the plasticmelt will flow through the die with minimum resistance and stagnation.

The heat transfer liquid 35 is of the type which has a relatively highthermal conductivity and a high boiling point. One liquid which has beensuccessfully used is sold under the trade name of THERMINOL FR-l and ismanufactured by the Monsanto Company, St. Louis, Missouri.

The improved apparatus and method of this invention has been presentedin connection with the making of polyvinyl chloride film using anassociated melt; however, it is to be understoodthat this invention maybe used to make film using any suitable viscoelastic material capable ofbeing extruded through an extrusion die.

it has also been found that temperatures of the die 1 l at mostmelbwetted surfaces are within one or two degrees Fahrenheit of thetemperature of the heat trans fer liquid 35 flowing through the die, asdetermined by actual measurements. Further, the meltwetted surfaces havebeen found to vary no more than roughly lus or minus 1 F. over the ran eof heat loads nor- P E discolorations, or holes.

In a typical application using the apparatus and method of thisinvention to make polyvinyl chloride film, the die 11 had an outletorifice diameter of inches and the die had 72 strand channels. Melt wasprovided from extruder 14 so that it entered the inlet 12 of die 11 at aflow rate of approximately 400 lbs. per hour, at a temperature ofapproximately 420 F., and a pressure of approximately 3,500 psi. Themelt extruding from the outlet orifice 13 was at a temperature ofapproximately 435 F.

It has been found by tests that the melt temperature rises in the diedue to dissipation of the pressure at the rate of about 6.5 F. per 1,000psi. Although some of the heat is lost to the cooler die surfaces, itwill be readily apparent that there is generally a need to provide forlocal cooling of the melt particularly in local hot spots near the dieoutlet. v

The heat transfer liquid 35 in this; application entered the die at atemperature of approximately 420 F. and a pressure of approximately 20psi and exited the die at approximately 420 F. and 15 psi.

The air 24 to the die was controlled so that it was heated by the heatexchanger 64 and entered the die 11 at a temperature of approximately390 F. and a flow rate of about 10 cfm. The air 24 exited the die atessentially the same temperature and flow rate. The air entered thebubble 25 at a temperature of approximately 380 F. and 10 cfm and exitedthe bubble at anestimated temperature of 200 F. and 10 cfm whereupon itwas reheated in the die so that it exited the die at the Whilepresent'exemplary embodiments of this inven' tion, and methods ofpracticing the same, have been il lustrated and described, it will berecognized that this invention may be otherwise variously embodied andpracticed within the scope of the following claims.

What is claimed is:

1. A die for extruding plastic film, said die having, an inlet passagefor receiving a plastic melt under pressure, a plurality of spacedchannels having inlet ends communicating with said passage and havingoutlet ends, a combining chamber in said die communicating with saidoutlet ends, said combining chamber having a pair of approximatelyhorizontally extending walls and an undulating peripheral surfaceextending substantially vertically between said walls, an annulus whichhasa' discharge orifice for extruding the melt in tubular form film andsaid annulus having a receiving end communicating with said combiningchamber, and a plurality of passages for a heat transfer fluid extendingthrough the die between said channel inlet ends and said combiningchamber, said plurality of passages for heat transfer fluid beingadapted for supplying heat transfer fluid in a controlled manneradjacent both sides of the plastic melt passing through the die, saidplurality of passages for supplying heat transfer fluid having inlet andoutlet passages passing between several adjacent pairs of the plasticmelt channels.

2. The die of claim 1, in which a pair of inlet and outlet passages forsupplying the heat transfer fluid pass between the same pair ofchannels.

3. The die of claim 1, in which the plurality of passages for heattransfer fluid include a plurality of passages adjacent the inner sideof the annulus and extend axially through the die generally parallel toeach other in the portion of the die surrounded by the annulus.

4. A die for making plastic film comprising, an inlet, a passage in saiddie communicating with said inlet, a stranding chamber in said dieadjoining the peripheral edge of said passage, a plurality of spacedsubstantially axially ex-tending channels in said die communicating withsaid stranding chamber and each having a discharge end, a combiningchamber in said die communicating with the discharge end of each spacedchannel, and an annular discharge orifice in said die communicating withsaid combining chamber, each of said chambers having a pair'of opposedapproximately horizontally extending walls and an undulating peripheralsurface extending substantially vertically between said walls, saidundulating surface being defined by a plurality of substantiallyV-shaped surfaces interconnected at their bases by substantially arcuatesurfaces with each arcuate surface being aligned with an associated wallportion of an associated channel, said V-shaped surfaces in saidstranding chamber providing smooth converging flow of plastic melt intosaid channels and said V-shaped surfaces in said combining chamberproviding smooth diverging flow of said melt into an annular passagecommunicating with said annular discharge orifice, said die beingadapted to receive a plastic melt in said inlet as a solid column whichis spread in said passage, separated into spaced strands in saidchannels, combined in said combining chamber to effectively minimizeuneven flow rates in different parts of said die, and extruded from saidorifice as a plastic tube having a substantially uniform thicknessthroughout.

5. A die as set forth in claim 4 and further comprising a fillet in eachof said chambers extending between one of its horizontal walls and bothits V-shaped surfaces and arcuate surfaces.

6. A die as set forth in claim 4 in which said inlet comprises a rightcircular cylindrical opening of extended length, each of said spacedchannels has a circular cross-sectional configuration along its fulllength, and said annular discharge orifice is defined by a pair ofcooperating spaced surfaces each having a right circular cylindricalconfiguration.

7. A die as set forth in claim 6 in which each of said spaced channelshas a substantially frustoconical configuration defined by a preciselyfinished inside surface to thereby assure smooth flow of melttherethrough.

8. A die for making plastic film comprising, an inlet, a passage in saiddie communicating with said inlet, a stranding chamber in said dieadjoiningthe peripheral edge of said passage, a plurality of spacedsubstantially parallel axially extending channels in said diecommunicating with said stranding chamber and each having a dischargeend, a combining chamber in said die communicating with the dischargeend of ,each spaced channel, said combining chamber having, a pair ofopposed approximately horizontally extending walls and an undulatingperipheral surface extending substantially vertically between saidwalls. and an annular discharge orifice in said die communicating withsaid combining chamber, said die being adapted to receive a plastic meltin said inlet as a solid column which is spread in said passage,separated into spaced strands in said substantially parallel channels,combined in said combining chamber to effectively minimize uneven flowrates in different parts of said die with said undulating surfaceassuring smooth diverging flow of plastic melt into an annular passagecommunicating with said orifice, and extruded from said orifice as aplastic tube having a substantially uniform thickness throughout.

9. A die for making plastic film comprising, an inlet, a passage in saiddie communicating with said inlet, a stranding chamber in said dieadjoining the peripheral edge of said passage, a plurality of spacedsubstantially parallel axially extending channels in said diecommunicating with said stranding chamber and each having a dischargeend, a combining chamber in said die communicating with the dischargeend of each spaced channel, and an annular passage in said diecommunicating with said combining chamber and terminating in an annulardischarge orifice, said die being adapted to recieve a plastic melt insaid inlet as a solid column which is spread in said passage, separatedinto spaced strands in said channels, combined in said combining chamberto effectively minimize uneven flow rates in different parts'of saiddie, and extruded from said orifice as a plastic tube having asubstantially uniform thickness throughout, said die comprising, threeportions in the form of a lower portion, an upper portion,

and a central portion, said inlet being defined as a right circularcylindrical bore in said lower portion, said passage being defined as aconically extending passage between said lower portion and said centralportion, said spaced channels being defined in said central portion andhaving longitudinal axes arranged in a substantially circular patternwhich is larger in diameter than the diameter of said annular dischargeorifice, said combining chamber being arranged within said circularpattern of said spaced channels and providing a flow path radiallyinwardly toward the center of said die into said annular passage, andsaid annular passage and annular discharge orifice being defined in saidupper portion.

10. A die for making plastic film comprising, an inlet, a passage insaid die communicating with said inlet, a stranding chamber in said dieadjoining the peripheral edge of said passage, a plurality of spacedsubstantially parallel axially extending channels in said diecommunicating with said stranding chamber and each having a dischargeend, a combining chamber in said die communicating with the dischargeend of each spaced channel, and an annular passage in said diecommunicating with said combining chamber and terminating in an annulardischarge orifice, said die being adapted to receive a plastic melt insaid inlet as a solid column which is spread in said passage, separatedinto spaced strands in said channels, combined in said combining chamberto effectively minimize uneven flow rates in different parts of saiddie, and extruded from said orifice as a plastic tube having asubstantially uniform thickness throughout, said spaced channels havinglongitudinal axes arranged in a substantially circular pattern which islarger in diameter than the diameter of said annular discharge orifice,said combining chamber being arranged within said circular pattern ofsaid spaced channels and providing a flow path radially inwardly towardthe center of said die intosaid annular passage, said die comprising abottom portion which is adapted to be supported in a fixed position anda rotatable portion comprising the remainder of said die and beingrotatable on said bottom portion.

11. A die for extruding plastic film having, an inlet passage forreceiving a plastic melt under pressure, a

plurality of spaced plastic melt conveying channels which have theirinlets communicating with said pas sage, an annulus which has adischarge orifice for extruding the melt in tubular form film and saidannulus having a receiving end communicating with the outlet ends ofsaid channels, said plastic melt conveying channels being arranged inparallel relation and having longitudinal axes arranged in asubstantially circular pat tern which is larger in diameter than thediameter of said discharge orifice, a plurality of passages for a heattransfer fluid extending through the die between the channel inlet endsand the annulus, said plurality of passages for heat transfer fluidbeing adapted for supplying heat transfer fluid in a controlled manneradjacent both sides ofthe'plastic melt passing through the die, saidplurality of passages for supplying heat transfer fluid having inlet andoutlet passages passing between several adjacent pairs of the plasticmelt convey.- ing channels.

12. A die for making plastic film comprising, an inlet, a passage insaid die communicating with said inlet, a stranding chamber in said dieadjoining the peripheral edge of said passage, a plurality of spacedsubstantially axially extending channels in said die communicating withsaid stranding chamber and each having a discharge end, a combiningchamber in said die communicating with the discharge end of each spacedchannel, and an annular passage in said die communicating with saidcombining chamber and terminating in an annular discharge orifice, saiddie having a lower portion, an upper portion, and a central portion,said inlet being defined as a right circular cylindrical bore in saidlower portion, said passage being defined as a conically extendingpassage between said lower portion and said central portion, said spacedchannels being defined in said central portion and having longitudinalaxes arranged in a substantially circular pattern which is larger indiameter than the diameter of said annular discharge orifice, saidcombining chamber being arranged within said circular pattern of saidspaced channels and providing a flow path radially inwardly toward thecenter of said die intosaid annular passage, and said annular spacedchannels has a decreasing cross-sectional area from its inlet to itsoutlet.

M. A die as set forth in claim 12 in which each of said spaced channelshas a substantially frustoconical configuration.

15. The die of claim 1 and further comprising passages for supplyingpre-heated gas continuously to inflate the tubular form film emergingfrom the orifice and passages for withdrawing cooled gas in the tubularfilm, said supplying passages comprising inlet passages extendingthrough the die inwardly between several pairs of adjacent spacedchannels and thence out of the portion of the die surrounded by theorifice, and said withdrawing passages comprising outlet passagesextending outwardly between several pairs of adjacent spaced meltchannels.

16. The die of claim 15 in which the gas outlet passages pass betweenpairs of channels other than those between which the gas inlet and heattransfer fluid inlet and outlet passages pass.

17. A die as set forth in claim 10 in which said rotatable portioncomprises a plurality of components fixed together by a plurality ofbolts with each bolt extending through aligned enlarged openings in saidcomponents to define an annular gas passage of extended length betweeneach bolt and said components, said gas passages being adapted to flowgas from said tubular form film substantially axially through said die.peripheral surface extending substantially vertically between saidwalls, and an annular discharge orifice in said die communicating withsaid combining chamber, said die being adapted to receive a plastic meltin said inlet as a solid column which is spread in said passage,separated into spaced strands in said substantially parallel channels,combined in said combining chamber to effectively minimize uneven flowrates in different parts of said die with said undulating surfaceassuring smooth diverging flow of plastic melt into an annular passagecommunicating with said orifice, and extruded from said orifice asaplastic tube having a substantially uniform thickness throughout.

asses:

1. A die for extruding plastic film, said die having, an inlet passagefor receiving a plastic melt under pressure, a plurality of spacedchannels having inlet ends communicating with said passage and havingoutlet ends, a combining chamber in said die communicating with saidoutlet ends, said combining chamber having a pair of approximatelyhorizontally extending walls and an undulating peripheral surfaceextending substantially vertically between said walls, an annulus whichhas a discharge orifice for extruding the melt in tubular form film andsaid annulus having a receiving end communicating with said combiningchamber, and a plurality of passages for a heat transfer fluid extendingthrough the die between said channel inlet ends and said combiningchamber, said plurality of passages for heat transfer fluid beingadapted for supplying heat transfer fluid in a controlled manneradjacent both sides of the plastic melt passing through the die, saidplurality of passages for supplying heat transfer fluid having inlet andoutlet passages passing between several adjacent pairs of the plasticmelt channels.
 2. The die of claim 1, in which a pair of inlet andoutlet passages for supplying the heat transfer fluid pass between thesame pair of channels.
 3. The die of claim 1, in which the plurality ofpassages for heat transfer fluid include a plurality of passagesadjacent the inner side of the annulus and extend axially through thedie generally parallel to each other in the portion of the diesurrounded by the annulus.
 4. A die foR making plastic film comprising,an inlet, a passage in said die communicating with said inlet, astranding chamber in said die adjoining the peripheral edge of saidpassage, a plurality of spaced substantially axially ex-tending channelsin said die communicating with said stranding chamber and each having adischarge end, a combining chamber in said die communicating with thedischarge end of each spaced channel, and an annular discharge orificein said die communicating with said combining chamber, each of saidchambers having a pair of opposed approximately horizontally extendingwalls and an undulating peripheral surface extending substantiallyvertically between said walls, said undulating surface being defined bya plurality of substantially V-shaped surfaces interconnected at theirbases by substantially arcuate surfaces with each arcuate surface beingaligned with an associated wall portion of an associated channel, saidV-shaped surfaces in said stranding chamber providing smooth convergingflow of plastic melt into said channels and said V-shaped surfaces insaid combining chamber providing smooth diverging flow of said melt intoan annular passage communicating with said annular discharge orifice,said die being adapted to receive a plastic melt in said inlet as asolid column which is spread in said passage, separated into spacedstrands in said channels, combined in said combining chamber toeffectively minimize uneven flow rates in different parts of said die,and extruded from said orifice as a plastic tube having a substantiallyuniform thickness throughout.
 5. A die as set forth in claim 4 andfurther comprising a fillet in each of said chambers extending betweenone of its horizontal walls and both its V-shaped surfaces and arcuatesurfaces.
 6. A die as set forth in claim 4 in which said inlet comprisesa right circular cylindrical opening of extended length, each of saidspaced channels has a circular cross-sectional configuration along itsfull length, and said annular discharge orifice is defined by a pair ofcooperating spaced surfaces each having a right circular cylindricalconfiguration.
 7. A die as set forth in claim 6 in which each of saidspaced channels has a substantially frustoconical configuration definedby a precisely finished inside surface to thereby assure smooth flow ofmelt therethrough.
 8. A die for making plastic film comprising, aninlet, a passage in said die communicating with said inlet, a strandingchamber in said die adjoining the peripheral edge of said passage, aplurality of spaced substantially parallel axially extending channels insaid die communicating with said stranding chamber and each having adischarge end, a combining chamber in said die communicating with thedischarge end of each spaced channel, said combining chamber having, apair of opposed approximately horizontally extending walls and anundulating PERIPHERAL SURFACE EXTENDING SUBSTANTIALLY VERTICALLY BETWEENSAID WALLS, AND AN ANNULAR DISCHARGE ORIFICE IN SAID DIE COMMUNICATINGWITH SAID COMBINING CHAMBER, SAID DIE BEING ADAPTED TO RECEIVE A PLASTICMELT IN SAID INLET AS A SOLID COLUMN WHICH IS SPREAD IN SAID PASSAGE,SEPARATED INTO SPACED STRANDS IN SAID SUBSTANTIALLY PARALLEL CHANNELS,COMBINED IN SAID COMBINING CHAMBER TO EFFECTIVELY MINIMIZE UNEVEN FLOWRATES IN DIFFERENT PARTS OF SAID DIE WITH SAID UNDULATING SURFACEASSURING SMOOTH DIVERGING FLOW OF PLASTIC MELT INTO AN ANNULAR PASSAGECOMMUNICATING WITH SAID ORIFICE, AND EXTRUDED FROM SAID ORIFICE AS APLASTIC TUBE HAVING A SUBSTNATIALLY UNIFORM THICKNESS THROUGHOUT.
 9. Adie for making plastic film comprising, an inlet, a passage in said diecommunicating with said inlet, a stranding chamber in said die adjoiningthe peripheral edge of said passage, a plurality of spaced substantiallyparallel axially extending channels in said die communicating with saidstranding chamber and each having a discharge end, a combining chamberin said die communicating with the discharge end of each sPaced channel,and an annular passage in said die communicating with said combiningchamber and terminating in an annular discharge orifice, said die beingadapted to recieve a plastic melt in said inlet as a solid column whichis spread in said passage, separated into spaced strands in saidchannels, combined in said combining chamber to effectively minimizeuneven flow rates in different parts of said die, and extruded from saidorifice as a plastic tube having a substantially uniform thicknessthroughout, said die comprising, three portions in the form of a lowerportion, an upper portion, and a central portion, said inlet beingdefined as a right circular cylindrical bore in said lower portion, saidpassage being defined as a conically extending passage between saidlower portion and said central portion, said spaced channels beingdefined in said central portion and having longitudinal axes arranged ina substantially circular pattern which is larger in diameter than thediameter of said annular discharge orifice, said combining chamber beingarranged within said circular pattern of said spaced channels andproviding a flow path radially inwardly toward the center of said dieinto said annular passage, and said annular passage and annulardischarge orifice being defined in said upper portion.
 10. A die formaking plastic film comprising, an inlet, a passage in said diecommunicating with said inlet, a stranding chamber in said die adjoiningthe peripheral edge of said passage, a plurality of spaced substantiallyparallel axially extending channels in said die communicating with saidstranding chamber and each having a discharge end, a combining chamberin said die communicating with the discharge end of each spaced channel,and an annular passage in said die communicating with said combiningchamber and terminating in an annular discharge orifice, said die beingadapted to receive a plastic melt in said inlet as a solid column whichis spread in said passage, separated into spaced strands in saidchannels, combined in said combining chamber to effectively minimizeuneven flow rates in different parts of said die, and extruded from saidorifice as a plastic tube having a substantially uniform thicknessthroughout, said spaced channels having longitudinal axes arranged in asubstantially circular pattern which is larger in diameter than thediameter of said annular discharge orifice, said combining chamber beingarranged within said circular pattern of said spaced channels andproviding a flow path radially inwardly toward the center of said dieinto said annular passage, said die comprising a bottom portion which isadapted to be supported in a fixed position and a rotatable portioncomprising the remainder of said die and being rotatable on said bottomportion.
 11. A die for extruding plastic film having, an inlet passagefor receiving a plastic melt under pressure, a plurality of spacedplastic melt conveying channels which have their inlets communicatingwith said passage, an annulus which has a discharge orifice forextruding the melt in tubular form film and said annulus having areceiving end communicating with the outlet ends of said channels, saidplastic melt conveying channels being arranged in parallel relation andhaving longitudinal axes arranged in a substantially circular patternwhich is larger in diameter than the diameter of said discharge orifice,a plurality of passages for a heat transfer fluid extending through thedie between the channel inlet ends and the annulus, said plurality ofpassages for heat transfer fluid being adapted for supplying heattransfer fluid in a controlled manner adjacent both sides of the plasticmelt passing through the die, said plurality of passages for supplyingheat transfer fluid having inlet and outlet passages passing betweenseveral adjacent pairs of the plastic melt conveying channels.
 12. A diefor making plastic film comprising, an inlet, a passage in said diecommunicating with said inlet, a stranding chAmber in said die adjoiningthe peripheral edge of said passage, a plurality of spaced substantiallyaxially extending channels in said die communicating with said strandingchamber and each having a discharge end, a combining chamber in said diecommunicating with the discharge end of each spaced channel, and anannular passage in said die communicating with said combining chamberand terminating in an annular discharge orifice, said die having a lowerportion, an upper portion, and a central portion, said inlet beingdefined as a right circular cylindrical bore in said lower portion, saidpassage being defined as a conically extending passage between saidlower portion and said central portion, said spaced channels beingdefined in said central portion and having longitudinal axes arranged ina substantially circular pattern which is larger in diameter than thediameter of said annular discharge orifice, said combining chamber beingarranged within said circular pattern of said spaced channels andproviding a flow path radially inwardly toward the center of said dieinto said annular passage, and said annular passage and annulardischarge orifice being defined in said upper portion.
 13. A die as setforth in claim 12 in which each of said spaced channels has a decreasingcross-sectional area from its inlet to its outlet.
 14. A die as setforth in claim 12 in which each of said spaced channels has asubstantially frustoconical configuration.
 15. The die of claim 1 andfurther comprising passages for supplying pre-heated gas continuously toinflate the tubular form film emerging from the orifice and passages forwithdrawing cooled gas in the tubular film, said supplying passagescomprising inlet passages extending through the die inwardly betweenseveral pairs of adjacent spaced channels and thence out of the portionof the die surrounded by the orifice, and said withdrawing passagescomprising outlet passages extending outwardly between several pairs ofadjacent spaced melt channels.
 16. The die of claim 15 in which the gasoutlet passages pass between pairs of channels other than those betweenwhich the gas inlet and heat transfer fluid inlet and outlet passagespass.
 17. A die as set forth in claim 10 in which said rotatable portioncomprises a plurality of components fixed together by a plurality ofbolts with each bolt extending through aligned enlarged openings in saidcomponents to define an annular gas passage of extended length betweeneach bolt and said components, said gas passages being adapted to flowgas from said tubular form film substantially axially through said die.peripheral surface extending substantially vertically between saidwalls, and an annular discharge orifice in said die communicating withsaid combining chamber, said die being adapted to receive a plastic meltin said inlet as a solid column which is spread in said passage,separated into spaced strands in said substantially parallel channels,combined in said combining chamber to effectively minimize uneven flowrates in different parts of said die with said undulating surfaceassuring smooth diverging flow of plastic melt into an annular passagecommunicating with said orifice, and extruded from said orifice as aplastic tube having a substantially uniform thickness throughout.